New P-T-X conditions for the formation of gem tsavorite garnet in the Voi area (southwestern Kenya)

Abstract

Tsavorite nodules-bearing deposits from southwestern Kenya are located in the Kurase Group, a metasedimentary unit of the Neoproterozoic Metamorphic Mozambique Belt. This unit is composed of graphitic paragneisses intercalated with metacarbonates and metaevaporites, surrounded by migmatites. The rocks underwent high grade metamorphism at 615–600 Ma. The main goal of this work is to link tsavorite formation to the metamorphic evolution of the Kurase Group. The new thermobarometric data indicate widespread granulite facies conditions at 800 ± 50 °C and 10 ± 1 kbar, with no significant difference between the tsavorite-bearing metasediments and the surrounding migmatitic gneisses. Pseudosection calculation for a tsavorite-bearing metasediment indicates that tsavorite grew close to peak-T conditions at around 800 °C. The tsavorite-bearing formations have not melted extensively despite the high-grade metamorphism, in contrast with the surrounding migmatites. The lack of partial melting is probably due to an enrichment in vanadium, chromium and titanium in the protoliths that have increased the stability field of micas toward high-T. We suggest that the primary source of V and Cr was the evaporite-bearing mudstones. Crystallisation of high grade V and Cr rich tsavorite occurred in a closed system with little or no strain, in the presence of molten salts and H2S-S8 fluids.